The present invention pertains generally to systems for controlling the locking and unlocking of a crane spreader assembly onto a container to be moved by the crane. More particularly, the present invention relates to a system whereby power is maintained to the locking or unlocking device after it is once actuated, independent of the subsequent state of container proximity sensing switches.
Prior art crane systems for moving cargo containers have provided proximity sensors for detecting when a spreader unit is in place on top of a container. Such sensors have usually been simple mechanical switches which are actuated when contact with a container is obtained, although more recent sensors detect proximity without the need for mechanical contact for switch closure. Locking of the spreader unit to the container is allowed only when all sensors are indicating that the spreader is in place with respect to the container. If one or more of these proximity sensors is not properly actuated to indicate that a container is in place, power is not allowed to be transferred to the locking device. Similarly, the proximity sensors must be all actuated to allow for unlocking of the spreader unit from the container. The difficulty with such systems is that once the crane has begun to lift the container, one or more of the proximity sensors will cease to be actuated, due to relative movement of the spreader with respect to the container. This movement inherently results from the massive locking structure required to transport the weight of a loaded container. In other words, although the spreader must be in intimate relation with the container to insure effective operation of the locking device, when the container is lifted, its weight as well as the necessary tolerance and natural wear of the locking device will result in a limited separation of the container from the spreader. Applicant has found that it is impractical, if not impossible, to design a system in which the proximity sensors are capable of both sensing the intimate relation necessary for effective locking of the spreader to the container and remaining actuated when the container is being transported by the spreader. The result according to the prior art, is that power to the locking device is decoupled during transportation of the container by the spreader.
The danger with the above condition is that it allows the solenoid controlled hydraulic valve customarily used to lock the spreader unit against the container to "float". That is, the controlling solenoid is no longer energized to insure that said valve remains in its lock position. Consequently, strong extraneous electromagnetic fields could cause the solenoid and valve to change state, thereby unlocking the container and allowing it to fall from the spreader. Such strong electromagnetic fields are not uncommon, since the valve and locking solenoid are positioned on the spreader unit, and the spreader unit is lowered into the holds of ships where other heavy duty electrical equipment, electrical power cables, degaussing coils, or the like, may be operating.
Thus, applicant has found that in order for the system to be safe, it is necessary to insure that the locking solenoid continues to be maintained in an actuated state, even though one or more of said proximity sensors is no longer actuated. A safe system must also insure that prior to enabling the unlocking function to occur, all of the proximity sensors must again be actuated by lowering the container onto a supporting surface, to reestablish thereby the intimate relation between the spreader and container.
Applicant has discovered that a temporary loss of system power in the crane will disrupt even an otherwise safe system and that means should also be provided for insuring that when power is restored, the system will be returned to an operative condition regardless of the state of the proximity sensors.